Ockham's Razor

The principle is sometimes stated as: do not multiply entities beyond necessity. More precisely, Ockham holds that when two theories equally explain the phenomena, the one that postulates fewer entities or types of entities is to be preferred. This is not a claim that simpler theories are more likely to be true — it is a claim about explanatory virtue: a theory that explains everything a complex theory explains, while requiring less ontological commitment, has achieved more with less and is therefore a better theory, all else being equal.

The razor's most important application in the Summa Logicae is against the realist theory of universals. Realists — following a certain reading of Plato and developed by Aquinas and Scotus — held that when we call two things by the same name (e.g., "white"), this is because they share a real property existing in both of them: whiteness as a universal instantiated in many particulars. Ockham argues that this postulates an unnecessary kind of entity. The name "white" can do all the explanatory work the universal was supposed to do: it refers to all white things by a convention established on the basis of resemblance, without requiring that there be any entity corresponding to it beyond the individual white things themselves.

The razor was taken up by early modern science as a methodological principle: prefer the simpler hypothesis that explains the data to the more complex one that does not explain it better. Newton's first rule of philosophising states it almost verbatim. In this form it shaped the development of mechanics, chemistry, and eventually biology — always preferring economy of explanation to extravagant postulation. Contemporary philosophy of science debates whether parsimony is a purely pragmatic virtue (simpler theories are easier to use) or whether it tracks something about the structure of reality (the world is, in some sense, genuinely simple). Ockham himself held the former view.

The phrase most commonly attributed to Ockham — "entities should not be multiplied beyond necessity" (entia non sunt multiplicanda praeter necessitatem) — does not appear verbatim in his work. What he actually wrote is that "plurality should not be posited without necessity" (pluralitas non est ponenda sine necessitate) and similar formulations. The principle's application in the Summa Logicae is against realism about universals, relations, and motion.